Devices having a balloon mounted at the distal end of a catheter are useful in a variety of medical procedures. A balloon reservoir may be used to deliver a biologically compatible fluid, such a radiologically opaque fluid for contrast x-rays, to a site within the body. Radial expansion of a balloon may be used to expand or inflate a stent positioned within the body. A balloon may also be used to widen a vessel into which the catheter is inserted by dilating the blocked vessel. For example, in the technique of balloon angioplasty, a catheter is inserted for long distances into blood vessels of extremely reduced diameter and used to release or dilate stenoses therein by balloon inflation. These applications require extremely thin walled high strength relatively inelastic balloons of accurately predictable inflation properties.
Depending on the intended use of the balloon and the size of the vessel into which the catheter is inserted, the requirements for strength and size of the balloon vary widely. Balloon angioplasty has perhaps the most demanding requirements for such balloons. The balloons should have uniformly thin walls and a small diameter in their unextended state, since the wall and waist thicknesses of the balloon limit the minimum diameter of the catheter distal end, and therefore determine the limits on minimum blood vessel diameter treatable by this method, as well as the ease of passage of the catheter through the vascular system. High balloon strength is required to enable the balloon to push open a stenosis and to avoid bursting of the balloon under the high internal pressures necessary to inflate the balloon at the site of the stenosis. Sufficient balloon elasticity is required to enable control of the inflated diameter and to allow the surgeon to vary the diameter of the balloon as required to treat individual lesions. To accurately control the balloon diameter, the elasticity of the balloon material must be relatively low. Small variations in pressure must not cause wide variations in balloon diameter.
In the past, PTA catheter balloons have been made from polymeric materials which gave balloons that may be broadly categorized into two groups: a) non-compliant balloons and b) compliant balloons.
Non-compliant balloons typically unfold to a nominal diameter and then stretch or expand only slightly (typically about 5% or less) beyond that diameter as the pressure is increased to burst. See Levy, U.S. Re 32,983, Wang U.S. Pat. No. 5,195,969 and Wang U.S. Pat. No. 5,330,428. All three patents describe biaxially oriented polyethylene terephthalate (PET) balloons. In comparison compliant balloons typically inflate to a nominal diameter and then continue to stretch or expand as the inflation pressure is increased until the strength of the balloon material is exceeded and the balloon bursts, producing a total expansion from nominal diameter to burst of above 5% but generally less than about 80%. See Becker U.S. Pat. No. 4,154,244 and Wang, et al, U.S. Pat. No. 5,556,383.
Balloon characteristics of particular distension and maximum pressure are influenced both by the type of polymer used in forming the balloon and by the conditions under which the balloon is radially expanded. Angioplasty balloons are conventionally made by radially expanding a parison of polymer material at a temperature above its glass transition temperature. For any given balloon material, there will be a range of distensions achievable depending on the conditions chosen for the radial expansion of the balloon.
Balloons have been formed of a wide variety of homopolymer and copolymer materials. The strength characteristics of the balloon may be provided by a single polymer layer or by several layers of polymer material. Balloons with multiple structural polymer layers may be produced by coextrusion, as described in WO 92/19316, U.S. Pat. No. 5,270,086 and U.S. Pat. No. 5,290,306, or by a tube-in-tube technique as described in U.S. Pat. No. 5,512,051; U.S. Pat. No. 5,587,125 and in copending U.S. application Ser. No. 08/611,664 filed Mar. 6, 1996 and PCT/US97/04061, filed Mar. 6, 1997.
In U.S. Pat. No. 5,270,086 it is proposed that a multilayer balloon could be made with an outer layer of a high tensile strength polymer and an inner bonding layer of a highly distensible polymer which had good melt bond and glue adhesion properties. Among the various materials proposed for the outer layer is "liquid crystal polymer". This reference, however, only exemplifies balloons in which the tensile layer is PET and provides no information whatsoever as to what types of liquid crystal polymers may be suitable, or how they may be processed to produce useful balloons.
In U.S. Pat. No. 5,306,246 balloons made of a blend of a crystallizable polymer and an additive that disrupts the crystalline structure are described. Use of liquid crystal polymers as such additives is described.
Various types of liquid crystal polymers are known. One type is a main chain LCP which has an orientational order composed of fairly rigid segments connected together end-to-end by flexible segments. A second type of LCP is a side chain LCP which has an orientational order composed of a single, completely flexible polymer with rigid segments attached along its length by short flexible segments. Nematic, chiral nematic and smectic phases, found in liquid crystals, have been also found in both main chain and side chain LCPs. Nematic LCPs are those in which the rigid sections tend to be oriented along a preferred direction. There is no positional order and the other parts of the LCP display no orientational or positional order. In chiral nematic (or cholesteric) LCPs, the preferred positional direction is not constant but rotates in a helical fashion. In smectic LCPs, the rigid, anisotropic sections of the monomer tend to position themselves in layers as they orient in the liquid crystal phase. Commercial liquid polymers include wholly or partially aromatic polyesters or copolyesters such as XYDAR.RTM. (Amoco) or VECTRA.RTM. (Hoechst Celanese). Other commercial liquid crystal polymers include SUMIKOSUPER.TM. and EKONOL.TM. (Sumitomo Chemical), DuPont HX.TM. and DuPont ZENITE.TM. (E.I. DuPont de Nemours), RODRUN.TM. (Unitika) and GRANLAR.TM. (Grandmont).
References describing liquid polymers include: U.S. Pat. Nos. 3,991,014, 4,067,852, 4,083,829, 4,130,545, 4,161,470, 4,318,842, and 4,468,364.
LCP polymer blends have been described in U.S. Pat. No. 4,386,174, 4,433,083 and 4,438,236. In U.S. Pat. No. 5,565,530, WO 93/24574 and WO 96/00752 compatibilized blends of liquid polymers are described.
Work by the inventors hereof with commercial liquid crystal polymers and with dry blends of such polymers with PET (i.e. blends produced in extruder by adding the individual polymer components to the extruder hopper) have demonstrated that liquid crystal polymers could not be readily fashioned into balloons for medical devices. Problems encountered included that the extruded tubing was so crystalline that it could not be subsequently blow molded into a balloon and that the extruded polymer was so brittle that the tubes broke up when handled.
To date it has not been suggested to use any type of polymer blend comprising a compatabilized blend of a crystallizable thermoplastic polymer and a liquid crystal polymer in a medical device balloon structure.